The prevalence of obesity is increasing at an alarming rate with the latest reports indicating that 27% of the adult US population is obese, and 34% are overweight. Although weight control research in the past two decades has significantly improved short-term treatment success, long-term weight loss maintenance has lagged behind with extremely high rates of recidivism. An integral part of behavioral treatment is the systematic observation and recording of eating and exercise behaviors, referred to as self-monitoring. Previous research has demonstrated a consistent relationship between self-monitoring and success in weight loss as well as in maintenance of weight loss. However, these studies have been based on selfmonitoring recorded in paper-and-pencil diaries that provide no indication of when the recording was done. The purpose of the proposed, descriptive ancillary study is to (1) describe electronically-recorded patterns of self-monitoring of eating and exercise behaviors by weight loss study participants using an Instrumented Paper Diary (IPD), a paper-and-pencil diary embedded with unobtrusive photosensors that detect when the diary binder is opened and closed and provide a time and date stamp for each action; (2) examine the relationship between electronically-validated adherence to self-monitoring and success in weight loss; (3) conduct in-depth interviews of the 6 most adherent and the 6 least adherent participants to explore their perspective on self-monitoring; and (4) determine if there are opportunities for targeted interventions to enhance adherence. Twenty-four subjects in the third cohort of the parent study will be randomly selected to use the IPD for three 2-week periods across the study's 18-month course; 12 participants will be selected for the interviews. This innovative study will extend the primary aim of the parent study by elucidating the role of actual self-monitoring in weight loss and by helPrincipal Investigatorng to identify interventions that may enhance selfmonitoring. In addition, the findings will add to the science of self-report in the field of weight loss treatment. [unreadable] [unreadable]